1. Field of the Invention
The invention relates to a four-wire communication line circuit comprising a first amplifier for one communication direction and a second amplifier for the other communication direction, control means for switching the relevant amplifier gain factors from a high value to a low value or the reverse, the four-wire line being coupled by a hybrid circuit to a two-wire subscriber line for two-way communication, and a transmission direction detection circuit for generating control signals for the control means depending on the direction in which net energy is transmitted.
2. Description of the Related Art
Such a four-wire connection line circuit is disclosed in Netherlands Pat. No. 136,420, in which it forms part of a telephone network for conference communication between a plurality of subscriber sets. A subscriber set has access to the other, similar, subscriber sets over a path comprising a two-wire line, a hybrid, the four-wire amplified line, a further hybrid and the other subscriber's two-wire line. By means of the transmission direction detection circuit, which is coupled to the two-wire line, it is determined in which direction the (net) speech energy flows. Thereafter, the amplifier in the four-wire line in that communication direction is rapidly switched to a high gain and the other amplifier which is included in the four-wire line for the other communication direction is simultaneously switched to a low gain. This circuit has the disadvantage that information from the two-wire line is required in order to switch the amplifiers in the four-wire line correctly. This requires additional signal wires. In addition, with electronic exchanges it is undesirable to take this information from the two-wire line as the signals are there still symmetrical relative to ground.